The objective of this proposal is to determine how changes in intracellular Ca2+, the ultimate signal of electrical activity, regulate cardiac membrane excitability. Dysregulation of Ca2+, however, contributes to cardiac arrhythmias. Given the potential pathophysiological consequences, it is not surprising that many regulatory mechanisms have evolved to control Ca2+ entry. These include Ca2+-dependent inactivation (CDI) of CaV1.2 L- type Ca2+ channels[unreadable]the focus of the previous proposal[unreadable]and mechanisms newly described by us that derived from our studies on CDI. Here we focus on novel means by through which Ca2+ influences gating of multiple channels to shape the cardiac action potential and influence excitability. Three specific aims are proposed. In the first aim, we build upon preliminary data showing that structural determination of the Nav1.5 C terminus (CT) is feasible by NMR spectroscopy. We will determine the atomic structure of the putative Ca2+ regulatory regions of Nav1.5, which will allow us to address the controversy about whether and how Nav1.5 is regulated by Ca2+. These experiments will also provide insight into atomic structure and explain function of the critical C terminus (CT) and III-IV intracellular linker of Nav1.5. Further, we will illuminate molecular mechanisms of specific inherited arrhythmogenic mutations through these structural studies. In the second aim, we recognize that CaM is but one Ca2+ binding protein (CaBP) that regulates ion channel function;in vivo Ca2+-sensitivity of ion channel function might reflect a convergence of multiple CaBPs acting upon different channel determinants. We explore novel regulation of CaV1.2 Ca2+ channels by KChIP2, a CaBP previously shown to modulate K+ currents, but not yet reported to control Ca2+ channels. The third aim focuses on how Ca2+ regulates ion channels through the activation of Ca2+-sensitive effectors, such as kinases. Building upon our previous demonstration that CaMKII interacts with CaV1.2 Ca2+ channels to control Ca2+- dependent facilitation (CDF), we propose to explore the consequences of a molecular mimicry model to understand how CaMKII and the Ca2+ channel accessory ?2a subunit interact to foster CDF. Together, results from these aims will define new means by which Ca2+ can feedback on multiple ion channels to regulate membrane excitability and show how dysregulation of these mechanisms is arrhythmogenic. Public Health Relevance Statement Ca2+ signaling is the final common pathway of electrical activity in the heart. Dysregulation of Ca2+ signaling contributes to cardiac arrhythmias. This proposal addresses how Ca2+ feeds back to regulate cardiac ion channels and examines the structure of critical domains in ion channels that are loci for inherited arrhythmogenic mutations.